At one time the motion picture industry was active in the development and promotion of various systems for the production of three dimensional motion pictures, particularly of the stereoscopic type. Various systems were developed for obtaining a three dimensional effect in viewing the motion picture projected from a moving motion picture film strip. Many of these systems involved the visual aspects of the projected motion picture; however, the use of a three dimensional or positional effect by the use of spaced individually controlled speakers was not generally employed. To obtain blending of the sound from individually controlled speakers positioned at selected locations in a theater, a system was developed wherein selected speakers were individually controlled by signals on carriers of differing frequency recorded directly on the film strip. This concept is described in U.S. Letters Pat. No. 2,714,633. By using multiple carrier control signals recorded on a motion picture strip, it was possible to control individual speakers which were driven by the same program sound signal so as to create the effect of blending the sound by volume variations so that the sound would appear to come from various locations in the theater. This prior concept of providing separate signals on different carriers for controlling individual speakers in a theater had distinct disadvantages. For one, only a limited number of speakers could be controlled since the carrier frequencies were limited by the availability of recording space and band width on the film strip. Thus, the use of the system disclosed in the prior patent identified above was not too successful in obtaining accurate control over several speakers in a theater to give the wide variety of positional sound effects which are necessary to accommodate the desired blending of the sound in a motion picture environment. In view of the limitations of carrier space on the film strip itself and the limited number of speakers which could be controlled by the system of the prior patent, this system was not used to any great extent in the motion picture industry. In addition, this prior patent involves no vehicle for controlling special effects in a theater such as lights, motor driven devices, and special effects sounds which cannot be carried conveniently on the sound track of a motion picture film.
In an attempt to create a blending of the normal sound and infrasound another system is disclosed in U.S. Letters Pat. No. 3,973,839. In this patent, low frequency signals are recorded on the film strip of a motion picture. The logic analysis of the existence and nonexistence of these two signals is used to control the theater speakers and special effects speakers to give the infrasound. Again, the signals are recorded on the film and are used directly to control the special effects noise generator to obtain infrasound within a theater or sound arena.
In the two systems described above, there is no convenient manner for controlling the blending of the sound within a theater when several speakers are employed alone or in combination with a plurality of separate and distinct special effects devices such as switches, motor control devices, etc.